In recent years, wireless communication technologies have developed rapidly, 3GPP standardization develops to Rel. 13, and key technologies cover wide configuration of small cells, carrier aggregation (CA), a 3D multi-antenna technology (such as multiple input multiple output (MIMO)), and LTE enabling at an unlicensed band (such as licensed-assisted-access), etc.
A sidelink communication mode refers to that a data packet needs not to pass a core network and a base station, and a communication link may be directly established between user equipments (such as UE 1 and UE 2) for communication. A sidelink communication may also be referred to as a device to device (D2D) communication. And a sidelink discovery process is generally performed before performing sidelink communication. For example, UE 1 needs first to discover whether UE 2 is near before transmitting information to UE 2 in the sidelink communication mode.
FIG. 1 is a schematic diagram of the sidelink communication, in which a case where two UEs (UE 1 and UE 2) both under coverage of a base station (such as an eNB) perform sidelink discovery or establish sidelink communication is shown. FIG. 2 is another schematic diagram of the sidelink communication, in which a case where one UE (UE 1) under coverage of a base station and another UE (UE 2) not under the coverage of the base station perform sidelink discovery or establish sidelink communication is shown. And FIG. 3 is a further schematic diagram of the sidelink communication, in which a case where two UEs (UE 1 and UE 2) neither under coverage of a base station perform sidelink discovery or establish sidelink communication is shown.
The sidelink communication mode may be used to expand coverage of a cell. Two examples of expanding coverage shall be given below with reference to FIGS. 1 and 2.
Scenario 1: as shown in FIG. 1, a UE (UE 2) is located at an edge of a cell, and its signals are relatively weak, which is very possible to leave coverage of the cell. Furthermore, there exists a UE (such as UE 1) nearby which is authorized by a network and has a function of relay. And in order to avoid service interruption, UE 2 may access to a base station via a relay UE 1 after discovering the relay UE 1, and proceed with performing normal service communication.
Scenario 2: as shown in FIG. 2, a UE (such as UE 2) is located in an area with no network coverage, and there exists a UE (such as UE 1) nearby which is authorized by a network and has a function of relay. In order to access to a network for communication, UE 2 may access to a base station via a relay UE 1 after discovering the relay UE 1 within coverage of a cell.
In the above-described scenarios, UE 2 may be referred to as a remote UE, which may be located within the coverage of a cell, or may be located out of the coverage of the cell. And a UE having a function of relay (such as UE 1) may be referred to a relay UE, may be referred to as a UE-network relay, and may also be referred to as a relay.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.